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MPGe -vs- MPD : The Real Figure of Merit?

Discussion in 'Energy, Environment, and Policy' started by scaesare, Jun 7, 2013.

  1. scaesare

    scaesare Active Member

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    MPGe -vs- MPD : The Real Figure of Merit?

    One thing I’ve not seen focused on regarding the Tesla Model S, and electric vehicles in general, is the actual cost of fuel… in this case electricity, as compared to gasoline. I call this “Miles Per Dollar”, or MPD for short.
    You can sometimes find this data, but often you have to derive it, because, as with other fuels, price varies with location… often significantly.

    What is more commonly found is “MPGe”, which is the EPA’s attempt to quantify how efficient a vehicle is in using the equivalent amount of energy from a gallon of gasoline. Unfortunately, because it doesn’t factor cost in to the equation (and rightly so, as mentioned above it would be hard to do so), I believe it gives people a false impression.

    For example, the Model S has an MPGe rating of 95. For the casual observer, they will assume that the Model S “gets about 2-3 times the mileage of a good economy petroleum-fueled car”. What that fails to account for is that THE COST TO ACQUIRE THE EQUIVALENT FUEL IS DRAMTICALLY LOWER IN JUST ABOUT EVERY CIRCUMSTANCE. This generality is based on the fact that, although electricity costs vary, the places with higher electrical costs also generally have higher fuel costs.

    So while it might take 2-3 gallons of gas (or diesel) to go 97 miles in the economy car , and only one gallon’s equivalent amount of electricity in the Model S, it will cost eight to twelve dollars (@$4/gal) versus three to four-and-a-half dollars (@ 10-15 cents/KWh).

    My own example: My Toyota 4Runner got 19 MPG. The Model S is rated at 5 times that (95 MPGe). You would expect it to then be 5 times less expensive to operate. Yet (for me) it’s much less. Driving the 4Runner 19 miles will consume 1 gallon of gas, costing me $4. Driving my Model S 19 miles will consume about 6KWh of energy, which with my state’s EV Time of Usage (ToU) metering plan of $0.055/KWh will cost me $0.33 That’s TWELVE times less fuel cost.

    Now of course, everybody’s costs vary, and that’s why it’s hard to slap a single rating on the car. I suspect, however, that for many (most?) folks the MPD is even more attractive than the MPGe rating would imply at first glance. And how much of a factor that makes in the value proposition depends on an individual’s driving habits, what type of vehicle they are transitioning from, etc...

    But when I talk to people, I’m sure point out what the energy costs are so they can see the value in considering it along with the other factors that drive the total cost of ownership.
     
  2. Doug_G

    Doug_G Lead Moderator

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    They use that because the cost of fuel and electricity vary widely over time and from location to location, whereas MPGe does not.
     
  3. Rodolfo Paiz

    Rodolfo Paiz P85 "Plug and Play"

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    My best results (easiest comprehension and acceptance) when explaining the difference to other people is to use unit cost (cents per mile) for both cases. My wife's car costs me $0.28/mile (an easy-to-grasp number for anyone, based on 15 mpg and $4.20/gallon in our case) whereas my Model S costs me $0.04/mile (based on 335 Wh/mile and $0.12/KWh). That's a 7x comparison, and anyone can viscerally FEEL the difference between them when I say that "for every $1 of electricity I spend in one car, I have to spend $7 on gasoline in the other."
     
  4. scaesare

    scaesare Active Member

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    Did you see where I said exactly that?

    :confused:
     
  5. phx182flyer

    phx182flyer P11051

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    For a comparison of electric cost vs gas cost for the Phoenix area (ok my cost...) check out : ElecPwr
     
  6. mknox

    mknox Well-Known Member

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    There's a number of ways to look at this.

    For instance, I recently took a 157 mile business trip that included parking at the meeting for about 7 hours. My energy usage was 283 Wh/mi for a total consumption of 44.5 kWh (all from the car's trip meter).

    My (off peak) electricity cost is $0.1252 * 44.5 kWh = $5.57 (cost of the trip)

    Around here, gas costs $1.20 / liter, so for $5.57 I could buy 4.64 liters = 1.2 US gallons.

    157 miles/1.2 gallons = 130.8 MPG

    So, a gas car would have to get 130.8 MPG (or 1.8 liters/100 km) to cost the same as the Model S in this scenario.

    To be fair, charging losses are not included because I did not specifically measure the power from my outlet, but I would think I'm still seeing well over 100 MPG in cost equivalence.
     
  7. irishstoutaz

    irishstoutaz Member

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    I like that comparison... for us it will even be better as there is supposedly an EV program with our utility company where the cost will be .02/kWh during non peak time (11pm - 5am). The car we are replacing was $.19/mile (20mpg $3.75/gallon) and if we get somewhere close to your Wh/mile we are looking at $.01/mile. :cool:


     
  8. Monto

    Monto Member

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    If you open the door of the car before you disconnect the charger, the dash will show how many kWh were replaced. It is not only charging looses, but idle loss. It might be 47 or 48 kWh in this case.
     
  9. igotzzoom

    igotzzoom Member

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    kWh/100 mi is probably the most objective yardstick, but it's going to take some time for people to break out of the MPG mindset.
     
  10. scaesare

    scaesare Active Member

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    I do similar, except use the whole dollar as the unit of comparison... "In my previous vehicle I could drive about 5 miles on a dollar's worth of gas, with the Model S, a dollar get's me 60 miles!"

    It so happens that my commute is just over 60 miles... so I often say that "My daily commute would cost me about $13-14 in gas... it costs me just over a dollar in electricity."
     
  11. aaron.s

    aaron.s Member

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    Guys -

    I'm with Rodolfo... Lowest (simple) denominator - cost per mile. It's the easiest by far for anyone to grasp...

    Right now for me, I'm at about 6.5 cents per mile. My Ford Fusion Hybrid I had before cost 12 cents per mile.

    Aaron


     
  12. Cattledog

    Cattledog Active Member

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    Yup. 17.5 cents/mile for my 5 series bimmer, 3.5 cents/mile for my S. 5-fold improvement.

    No wonder the franchised dealers are worried, that's not even the main reason to drive the car!
     
  13. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    I just tell them it has cost me about $140 to drive the 5,300+ miles I have on the car so far.
     
  14. scaesare

    scaesare Active Member

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    Good replies, all. I think it's a great "awareness" opportunity for us all to underscore the real world operating cost, and thus savings, of these vehicles.

    I expect to save on the order of $500/month driving the Model S as opposed to my other vehicles. That's $36K ofer the life of the loan. Or $48K over the life of the 8-yr battery warranty.

    My $93K initial acquisition cost looks much more attractive when you consider TCO from that standpoint (and the $7,500 fed tax certainly doesn't hurt.).
     
  15. mknox

    mknox Well-Known Member

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    Really? I didn't know that! Thanks, I'm going to check that out. I do have an energy monitor on my charging circuit, but because I do a bit of "opportunity charging" when I'm out and about, that never really gave me a true picture of input power.

    - - - Updated - - -

    Maybe I'm just thick in the head, but I like MPG or MPGe. The kWh or gallon is a unit I can visualize. I know how much it costs. What I want to know is how far I can go on that easy to identify with unit of energy. Here in Canada they use liters/100km which is just awkward to think about. I never drive "exactly" 100 km, so how is that helpful?
     
  16. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Beats me. km/litres seems much more logical. Then again, since when did logic ever have anything to do with government?
     
  17. derekt75

    derekt75 Member

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    I think energy per distance is the important quantity, though, since distance is fixed.
    If you improve a 15 mpg car to 20 mpg, is that better or worse savings than improving a 30 mpg car to 50 mpg?

    Restated, it's clearer:
    if you improve a 67g/1000m car to 50g/1000m, is that better or worse savings than improving a 33g/1000m car to 20g/1000m?
    well, the first one saves 17 gallons per 1000 miles while the 2nd example only saves 13 gallons.
    You'll save more dollars or carbon dioxide by improving the 15 mpg car by 5 mpg than by improving the 30 mpg car by 20 mpg.

    The only problem with using energy per distance is that typical gasoline units and distances don't give you numbers between 1 and 100, and non-scientists don't like numbers like 0.017.


    I think it's nice that my car prefers to report Wh per mile rather than miles per kWh.

    Anyway, I think energy per distance is generally preferable to distance per energy.
    I do agree that using 100km as the denominator is ugly.
    I'd prefer mL/km, but I can see people complaining that they don't know what a mL is compared to a L.

    - - - Updated - - -

    oh, and regarding the subject of the thread,
    I think cents per mile is interesting, but I also think pounds of CO2 per mile is interesting. Again, there's a lot of location to location variation, but I think it would be nice. MPGe does seem like an odd calculation.
     
  18. stopcrazypp

    stopcrazypp Well-Known Member

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  19. dhrivnak

    dhrivnak Active Member

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    Be careful as I am sure your cost is actually significantly higher. I think the W/mile on the display is what the motor is using. One loses about 15% in charging efficiency so you would use 7KWH to make up the 6 KWH used. Then there is the vampire losses. I am reading the Model S loses about 3.2KWH/day just sitting there. Cars with their sealed gas tanks can go months without any loss of fuel.

    A Model S IS much less to operate than an ICE car. But let's not overstate the savings. The EPA number of 5X more efficient is already more than most people wrap their mind around.
     
  20. mknox

    mknox Well-Known Member

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    Who cares? I know what a gallon of gas is and how much it costs. What I need to know is how far that will take me. In either case, I know the higher number will take me further for the same cost. In real life, I wouldn't care about which example is a better savings expressed as a percentage.
     

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